This invention relates to the packaging of electronic components and, more particularly, to a package with reduced bending stress due to temperature change.
Some microelectronic package designs employ a thick plastic body molded on a thin substrate. A good example is a package in which a thin ceramic substrate is overmolded with an epoxy compound which encapsulates microelectronic components surface-mounted on the substrate. This substrate, since it is made of a brittle material, is prone to failure, because of the thermal contraction mismatch of the molding compound with the ceramic material as the package cools from its molding temperature to room temperature conditions.
The situation can be improved in many ways. For example, as disclosed in U.S. Pat. No. 5,627,407, an additional "surrogate" material (i.e., a material not needed from the standpoint of the normal operation of the package) can be applied to the outer surface of the substrate to "balance" the package bowing. Clearly, such a surrogate layer should be thin (in order not to interfere with the normal function of the electronic component, and not to make the package thicker than necessary), have a high coefficient of thermal expansion (CTE) and a high Youngs's modulus (to be effective) and, in addition, should be able to withstand high tensile stresses on both a short term and a long term basis. One could select, for example, a thermoplastic sheet that softens at the molding temperature and bonds well to the substrate, or a rigid material with an adhesive layer that bonds to the substrate during molding. This approach, based on the employment of a polymeric material molded concurrently with the "main" package, has, however, the following major shortcomings:
There are not too many polymer materials which have high CTE, high Young's modulus, high adhesive and cohesive strength and, at the same time, have a molding Temperature the same as the "basic" molding compound; and PA1 There is a concern that this material can fail in the long-run, either adhesively or cohesively, because of acing, high tensile stresses, moisture-absorption, etc.
It would therefore be desirable to provide an alternative solution to the problem of reducing the package bow using a surrogate material.